Vanadium nitrogen steel



March 16, 1965 G. F. MELLOY ETAL 3,173,782

VANADIUM NITROGEN STEEL Filed June 15, 1962 2 Sheets-Sheet l Actual Cale.

1 I I l 1 I S R a 9 J, aJwmadwq 00 1 5004 ondw a asnazau 3 000/ w -ws P/QIA 4 w INVENTORS Geprge E Mel/0y.

Joseph D. Dem/Leon Jr Bernard J Fischer United States Patent 3,173,732 VANADEUM NITROGEN'STEEL George F. Melloy, Joseph D. Dennison, 511., and Bernard J.

Fischer, Bethlehem, Pa, assignors, by mesne assignments, to Bethlehem dteei (Tarpon-anion, a corporation of Delaware Filed June 13, 1962, Ser.*No.-2tl2,241 6 Claims. (Cl. 75-123) This application is a continuation-in-part of the copending application of George F. Melloy et al., Serial No. 47,166, filed August 3, 1960, now abandoned.

This invention relates broadly to high strength steels and more particularly to semi-killed carbonmagnanese-vanadium-nitrogen steels having exceptionally high yield strengths in the as-rolled condition.

It has not been possible in the past toproduce semikilled steels having high yield strengths in the as-r-olled condition, even though such steels have been widely desired because of the far greater yield per ingot, superior surface, and much lower cost than killed steels. In addition to having relatively low yield strengths, semi-killed steels in the past have also been characterized'by coarser grain siZe and relatively higher impact transition temperatures than killed steels.

It is an object of this invention to provide novel semikilled carbon-rnanganese-vanadium-nitrogen steels having high yield strengths and low impact transition temperatures in the as-rolled condition.

It is a further object of the invention to provide novel fine grained semi-killed carbon-manganese-vanadiurn-ni- 'trogen steels having as-rolled yield strengths and impact properties equal to or better than those of some of the much more costly killed steels.

It is well known that vanadium can be added to semi- :killedsteels to increase the strengths thereof, but the increase in strength per unit of added vanadium is relatively small. Inasmuch as vanadium is a deoxidizer, not more than about vanadium can be added to steel if such steel is to have the advantages inherent in semi-killed steels. For these reasons high strength carbon-manganesevanadium steels cannot be made as semi-killed steels.

It is also well known that nitrogen can be added to increase the as-rolled strengths of steels. ,However, nitrogen also increases the impact transition temperatures markedly. Thus, nitrogen would appear to be an undesirable element in steel it said steel is to have a combination of high strength and good impact properties.

We have discovered that the addition of the combination of vanadium and nitrogen in specified amounts to semi-killed carbon-manganese steels greatly increases the as-rolled yield strengths of said steels, provides fine grain size, and maintains good impact properties while retaining the good yields and superior surfaces of semi-killedtsteels. These steels have excellent properties for use as construo tion materials, e.g. structural elements, reinforcing bars, and high-strength plates. We have also discovered that normalizing our steels within specific temperature ranges markedly improves the impact transition temperatures while maintaining or only slightly decreasing the yield strengths of such steels, depending upon the normalizing temperatures.

The invention will be more apparent from the following description taken with theadrawings, in which FIG. 1 is a graph showing the effects of vanadium and nitrogen, individually and combined, upon'the as-rolled yield strengths of semi-killed carbon-manganese steels.

FIG. 2 is a graph showing the effects of vanadium and nitrogen, individually and combined, upon the impact transition temperatures of semi-killed carbon-manganese steels.

3,l'i3,732 Patented Mar. E6, 196

FIG. 3 is a graph showing the effects of normalizing on the as-rolled yield strengths and impact transition temperatures of carbon-magganese-vanadium steels and steels of the invention.

The unexpected properties of the steels of the invention are obtained by the addition to semi-killed carbon-manganesesteelsof vanadium and-nitrogen inamounts sufficient to resultin a steel having a vanadium content of from about 02% to .20% and nitrogen in amounts of from about 008% to 024%, and preferably vanadium in amounts of from about 04% to 08% and nitrogen in amounts of from about 008% to 015%.

Our broad rangcof compositions isas follows:

Percent C 12-50 Mn .60-2.00 V .02.20 N 008-024 Si .07 max.

balance essentially iron, and residual impurities which do not adversely aliect the properties of the steel. Various amounts of these elements within the broad ranges stated are permissible as long as they are presentin amounts which do not kill the steel or otherwise adversely affect the novel properties of steels of the invention.

The carbon content ,of our steels should be at least about 312%. Lowering the carbon content below about .12% significantly decreases the strength of the steel. We have found that increasing the carbon content above about 50% is not advantageous as suchlarger amounts increase the strength of the steel only slightly while raising the impact transition temperatures significantly. In addition, larger amounts of carbon may kill the steel. The high strength steels of the invention contain at least about .60%

-manganese. Steels containing less than about;.60% manganese are low in strength and are apt to have a poor surface, i.e. tears may occur during processing. Increasing the manganese above about 2.00% may kill the steel, as may silicon above about 07% and vanadium above 10%. If the steelcontains nomore than residual amounts of vanadium and nitrogen there is substantially no improvement in the properties of the steel. -An improvement in impact and strength propertiesbegins to be apparent at minimum levels of about .02%-vanadium and about 008% nitrogen. Nitrogen in amounts in excess of about 024% may seriously damage the surface of the steel, i.e.bleeders and'other defects may occur.

The as-ro'lled yieldstrengths'and impact transition temperatures of our steels vary somewhat due to differences in carbon and manganese content. For example, increasing the carbon raises the yield strength and the impact transition temperature and --increasing the manganese raises the yield strength and lowers the impact transition temperature. A good balance of yield strength and impact properties. can beobtained by using a range including carbon .15 to .25 manganese 0.60 to 1.50%, vanadium .04 to 08%, nitrogen..008 .to, 015% and other elements in amounts insufilcient to kill the steel.

To demonstrate the effectiveness of the addition of the combination of .vanadium and nitrogen in raising the strength. levels of. asemisskilled. carbon-manganese steel in comparison with the effectiveness of vanadium alone, samples of three carbon-manganese steels were rolled to a thickness of .420" and tested. Sample No. 1 had no vanadium or nitrogen additions, sample No.2 had only a vanadium addition, and sample'No.'3 had vanadium and nitrogen additions to provide a steel having these elements within the ranges, of the invention. To demonstrate how the properties of steels having vanadium and nitrogen can be varied by changes of analysis, an addi- TABLE I FIGURES 1 and 2 show the unexpected elfects of the combination of vanadium and nitrogen upon the as-rolled Effect of vanadium and nitrogen additions n the (lS-IOlled properties of semi-killed carbon-manganese steel (.420" nominal thickness) Composition (percent) Ft. Lb.

I Charpy Y.S., 2% TS. Percent Elong, V-Notch Steel Sample No. ofiset (p.s.i.) Red. of Percent Impact 0 Mn Si V N (p.s.i.) Area in 2 Transition Temperature, F.

1 Residual Am0untsN0ne Added.

It is evident from a comparison of the steel samples in Table I that carbon-manganese steels having both vanadium and nitrogen additions exhibit a remarkable increase in as-rolled yield strength and little or no increase in impact transition temperature. Considering the increase in yield strength of sample No. 3, the rise in impact transition temperature is relatively small, while sample No. 4 combines a high yield strength with an excellent impact transition temperature.

The grain sizes of the above steels were measured. Samples 2, 3 and 4 were finer grained than sample No. l, as is shown below.

No. of ferrite Steel sample No.: grains/in. at 100x 1 125-150 3 and 4 200-225 It is evident that vanadium refines the grain of carbonmanganese steels. It can further be seen that the combination of vanadium and nitrogen results in a finer grain than that of steel to which vanadium only is added. I

A second series of steel samples Was rolled to a thick ness of .500 and tested. As shown in Table II below, sample No. 5 was a semi-killed carbon-manganese-vanadium steel and samples 6 and 7 were similar steels containing 012% and 017% nitrogen, respectively. The high gain in yield strength with relatively small loss in ductility and impact properties of the nitrogen treated steels over the untreated steelis clearly shown.

TABLE II yield strengths and impact transition temperatures, respectively, of semi-killed carbon-manganese steels. To obtain the data for FIG. 1, a heat of steel containing .12% carbon, .90% manganese, and 006% residual nitrogen was melted and four ingots were teemed therefrom.

The first ingot was used as a control ingot and no additions of vanadium or nitrogen were made thereto. A total of 06% vanadium was added to the second ingot, .007% nitrogen to the third ingot, and 06% vanadium plus 007% nitrogen to the fourth ingot. The steel contained sufiicient residual nitrogen so that, after the nitrogen additions, the third and fourth ingots had a nitrogen content of 013%. All four ingots were then rolled to /a" plate using identical rolling practice.

As is shown in FIG. 1, the addition of 06% vanadium to the base material increased the as-rolled yield strength of the steel by about 4,500 psi. The addition of 007% nitrogen to the base material increased the as-rolled yield strength of the steel by about 4,000 p.s.i. Thus, in view of the individual effects of nitrogen and vanadium on the as-rolled yield strength, it would be expected that the addition of both .06% vanadium and 007% nitrogen to the base material would result in an increase in the asrolled yield strength of the steel over that of the control analysis of about 8,500 p.s.i. However, the yield strength of the steel from the fourth ingot was about 14,000 p.s.i. higher than that of the steel from the first or control nominal thickness) Composition (percent) 15 Ft. Lb.

Charpy Y.S., 2% TS. Percent Elong., V-Notch Steel Sample No. ofiset (p.s.i.) Bed. of Percent Impact 0 Mn V N (p.s.i.) Area in 2" Transition Temperature,

1 Residual AmountNone Added.

The properties of the steels shown in Tables I and II ingot. It is clear from the foregoing that the strengthenwere made employing standard hot mill practice which included starting rolling at about 2250 F. and finishing above about 1700 F.

FIG. 2 shows the rise in impact transition temperature, for each 10,000.p.s.i. increase in as-rolled yield strength, obtained by: (1) the addition of vanadium alone; (2) the addition of nitrogen alone; and (3) the addition of both vanadium and nitrogen; to semi-killed carbon-manganese steels. The datawere obtained by means of standard tests on substantially identical steels, insofar as composition, plate thickness, etc, were concerned. The tests employed were the well-known tensile and Charpy V-notch impact tests. When vanadium alone was added to the base material the impact transition temperature rose-by about 55 F. for each 10,000 p.s.i. increase in the as-rolled yield strength of the steel. When nitrogen alone was added to the base material, the impact transition temperature rose by about 69 F. for each 10,000 p.s.i. increase in the as-rolled yield strength of the steel. Thus, in view of the individual effects of vanadium and nitrogen on impact transition temperatures, one would expect that upon the addition of a combination of vanadium and nitrogen to the base material, the rise in impact transition temperature would be the average due to the effects of vanadium and nitrogen separately, i.e. about 62 Fpfor each 10,000 p.s.i. increase in the as-rolled yield strength of the steel. However, the impact transition temperature of the carbonmanganese-vanadium-nitrogen steel actually rose by only about 40 F. for each 10,000 p.s.i. increase in as-rolled yield strength. This small rise in impact transition temperature is quite unexpected, and very important for the Composition (percent) Ferrite Grains-Per Square Inch at X100 Type Steel 0 Mn V N As-Rolled Normalized C-Mn 17 1. 20 1 002 1 004 130/150 1 100/120 G-Mn-V 17 l. 22 061 1 .004 1751200 150075 C-Mn-V-N .17 '1. 23 0S2 019 200/225 1 500/550 1 Resid11al*A1nountsNone Added.

Specific proper-ties obtained-by normalizing-the carbon-manganese-vanadium-nitrogen steels of -the=ii1vention, i;e. steel samples 10-and 11, arev compared with normalized carbon-manganese steel sample 8and carbon- -manganese-vanadium steel sample'9 in Table IV below.

TABLE IV Normalized Properties C-Mn, C-Mn-V, and

C-Mn-V-N steels Composition (percent) 15 Ft. Lb.

Plate 1.8., 2% Elong. Gharpy V- Steel Thickoilset percent RA. Notch Impact No. ness (p.s.i.) (p.s.i.) in 2" (percent) Transition 0 Mn V N Tempelr ature,

1. 20 .002 .004 .420" 33,000 i 05,500 as. 0 72. 7 -s 1. 22 .001 .004 .420" 40, 000 71, 500 35. s 72. s -40 1. 23 .062 .019 .420" 00,500 78,500 as. 5 71.4 -70 1. 36 .054 .012 390 i 57, 500 77, 500 .33. 5 72. 2

1 Residual AmountsNone added.

reason that prior semi-killed steels could not be greatly strengthened without a correspondingly large rise in transition temperature.

The carbon-manganese-vanadium-nitrogen steels referred to in FIGS. 1 and 2 were made according to a semi-killed practice which would normally result in a coarse grained steel. However, the steels were fine grained.

Experiments were made with special rolling practices, and it was found that by starting rolling at about 1900" F. and finishing below 1700 F., preferably at 1500 F. or lower, still greater improvements in as-rolled yield strengths and impact transition temperatures were obtained. A comparison of properties obtained by standard and special rolling practices is shown in Table III.

TABLE III 'FIG. 3 graphically shows the effects of normalizing on the yield strengths and'transition temperatures of carbon-manganese-vanadium vanadium-nitrogen steels.

and The properties of the steel to carbon-manganese- 59 which no nitrogen'was added, herein called steel A, are shown in dotted lines and the properties of the steel of the invention, herein called steel B, are shown in solid Efiect of hot mill practice on [is-rolled properties of 0.20% C, 1.40% Mn, 0.06% V, 0.012% N semikl'lled steel (.500 nominal thickness) The composition of steel B was:

Percent p N .009 Fe Balance Both steels were rolled to plate by identical hot mill practice and the properties thereof were determined in the as-rolled condition and in the normalized condition for normalizing temperatures of 1650" F., 1700 F., and 1750 F., respectively.

It can be seen from FIG. 3 that the as-rolled yield strength of steel B is considerably higher than that of steel A, while the impact transition temperature of steel B is only slightly higher than that of steel A. Normalizing steel B at 1650 F. decreased its transition temperature by 110 F. and decreased its yield strength by 7000 p.s.i., while normalizing steel A at the same temperature decreased its transition temperature by only 25 F. and decreased its yield strength by 3500 p.s.i. A comparison of the properties of the steels normalized at 1650 F. shows that steel B has a 5000 p.s.i. higher yield strength and a 70 lower transition temperature than steel A.

Normalizing steel B at 1700 F. and 1750 F. resulted in substantial improvements in transition temperatures over the as-rolled property, although said improvements were less than that which resulted upon normalizing at 1650 F. However, the yield strength of steel B decreased by only 3500 p.s.i. upon normalizing at 1700 E, and the yield strength actually increased by 1000 p.s.i. upin normalizing at 1750 F.

On the other hand, normalizing steel A at 1700 F. resulted in no improvement in transition temperature over that of the as-rolled steel with a slight decrease in yield strength. Normalizing steel A at 1750 F. resulted in an increase in impact transition temperature over that of the as-rolled steel while the yield strength decreased slightly.

It is to be noted that the highest transition temperature of the normalized steel of the invention is lower than the lowest transition temperature of the normalized carbonmanganese-vanadium steel. In addition, the lowest yield strength of the normalized steel of the invention is higher than the highest yield strength of the normalized carbonmanganese-vanadium steel.

The steels of the invention may be made by standard steelmaking practices. The vanadium and the nitrogen may be added to the steel either in the ladle or in the ingot.

While we have thus described our invention in considerable detail, we do not wish to be limited narrowly to the exact ancl specific particulars disclosed, but we may also use such substitutes, modifications or equivalents thereof as are included within the scope and spirit of the invention or as pointed out in the appended claims.

We claim:

1. A semi-killed steel consisting essentially of about .12% to about .50% carbon, about .60% to about 2.00% manganese, at least about .02% vanadium, about .008% to about 024% nitrogen, balance iron.

2. A semi-killed steel consisting essentially of about .12% to about .50% carbon, about .60% to about 2.00% manganese, about .02% to about 20% vanadium, about .008% to about 024% nitrogen, balance iron.

3. A semi-killed steel consisting essentially of about .15% to about 25% carbon, about .60% to about 1.50% manganese, about .04% to about .08% vanadium, about .008% to about .015 nitrogen, balance iron.

4. A normalized semi-killed steel consisting essentially of about .12% to about .50% carbon, about .60% to about 2.00% manganese, at least about .02% vanadium, about .008% to about .024% nitrogen, balance iron.

5. A normalized semi-killed steel consisting essentially of about .12% to about .50% carbon, about .60% to about 2.00% manganese, about .02% to about .20% vanadium, about 008% to about .024% nitrogen, balance iron.

6. A normalized semi-killed steel consisting essentially of about .15% to about 25% carbon, about .60% to about 1.50% manganese, about .04% to about .08% vanadium, about 008% to about .0l5% nitrogen, balance lI'OI'l.

OTHER REFERENCES Making, Shaping and Treating of Steel, United States Steel, Seventh Edition, Jan. 28, 1958, page 364.

DAVID L. RECK, Primary Examiner.

MARCUS U. LYONS, Examiner. 

2. A SEMI-KILLED STEEL CONSISTING ESSENTIALLY OF ABOUT .12% TO ABOUT .50, CARBON, ABOUT .60% TO ABOUT 2.00% MANGANESE, ABOUT .02% TO ABOUT .20% VANADIUM, ABOUT .008% TO ABOUT .024% NITROGEN, BALANCE IRON. 